Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Polymeric membranes have proven to operate successfully in industrial gas separations such as in the separation of nitrogen from air and the separation of carbon dioxide from natural gas. Cellulose acetate (CA) is a polymer currently being used in commercial gas separation. For example, UOP LLC's Separex™ CA membrane is used extensively for carbon dioxide removal from natural gas. Nevertheless, while they have experienced commercial success, CA membranes still need improvement in a number of properties including selectivity, permeability, chemical and thermal stability. Natural gas often contains substantial amounts of heavy hydrocarbons, aromatics, and water, either as an entrained liquid, or in vapor form, which may lead to condensation within membrane modules. The gas separation capabilities of polymeric membranes are affected by contact with liquids including hydrocarbons and water. The presence of more than modest levels of hydrogen sulfide, especially in conjunction with water and heavy hydrocarbons, is also potentially damaging. Therefore, precautions must be taken to remove the entrained liquid water and heavy hydrocarbons upstream of the membrane separation steps. Another issue of polymeric membranes that still needs to be addressed for their use in gas separations is the plasticization of the polymer by condensable gases such as carbon dioxide and propylene that leads to swelling of the membrane as well as a significant increase in the permeability of all components in the feed and a decrease in the selectivity of the membranes. For example, the permeability coefficient of CO2 in polymeric membranes begins to increase when the pressure is above a certain level due to the onset of plasticization by the CO2. A high concentration of sorbed CO2 leads to increased segmental motion, and, consequently, the transport rate of the penetrant is enhanced. The challenge of treating gas, such as natural gas, that contains relatively large amounts of CO2, such as more than about 50%, is particularly difficult.
Polymeric membrane materials have been found to be useful in gas separations. Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones, etc.) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (See, for example, U.S. Pat. No. 6,932,589). The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
The relative ability of a membrane to achieve the desired separation is referred to as the separation factor or selectivity for the given mixture. There are however several other obstacles to use of a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions. One such obstacle is permeation rate. One of the components to be separated must have a sufficiently high permeation rate at the preferred conditions or else extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Another problem that can occur is that at conditions where the permeability is sufficient, such as at elevated temperatures or pressures, the selectivity for the desired separation can be lost or reduced. Another problem that often occurs is that over time the permeation rate and/or selectivity is reduced to unacceptable levels. This can occur for several reasons. One reason is that impurities present in the mixture can over time clog the pores, if present, or interstitial spaces in the polymer. Another problem that can occur is that one or more components of the mixture can alter the form or structure of the polymer membrane over time thus changing its permeability and/or selectivity. One specific way this can happen is if one or more components of the mixture cause plasticization of the polymer membrane. Plasticization occurs when one or more of the components of the mixture act as a solvent in the polymer often causing it to swell and lose its membrane properties. It has been found that polymers such as cellulose acetate and polyimides which have particularly good separation factors for separation of mixtures comprising carbon dioxide and methane are prone to plasticization over time thus resulting in decreasing performance of these membranes.
Some new high-performance polymers such as new polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole exhibit high ideal selectivities for CO2 over CH4 when measured with pure gases at modest pressures in the laboratory. However, the selectivity obtained under mixed gas, high pressure conditions is much lower than the calculated ideal level. In addition, gas separation processes based on glassy solution-diffusion membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer represented by the membrane structure swelling and a significant increase in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases.
Thus, there is a critical need for new high-performance membranes that will provide and maintain adequate performance under conditions of exposure to organic vapors or liquids, high concentrations of acid gases such as CO2 and hydrogen sulfide, and water vapor that are commonplace in natural gas treatment.
Conventional methods for stabilizing polymeric membranes involve either annealing or cross-linking. Cross-linking is a useful method to suppress the polymer membrane plasticization. Polymer membrane cross-linking methods include thermal treatment, radiation, chemical cross-linking, and UV-photochemical processes. Cross-linking offers the potential to improve the mechanical and thermal properties of a membrane. Cross-linking can be used to increase membrane stability in the presence of aggressive feed gases and to simultaneously reduce plasticization of the membrane. Normally, cross-linked polymer membranes have a high resistance to plasticization, but their other properties such as permeability and selectivity are much less than desired.
US 2005/0268783 A1 disclosed chemically cross-linked polyimide hollow fiber membranes prepared from a monoesterified polymer followed by final cross-linking after hollow fiber formation.
U.S. Pat. No. 7,485,173 disclosed UV cross-linked mixed matrix membranes via UV radiation. The cross-linked mixed matrix membranes comprise microporous materials dispersed in the continuous UV cross-linked polymer matrix.
U.S. Pat. No. 8,337,598 disclosed a thin film composite hollow fiber membrane with a core layer and a UV-crosslinked polyimide polymer sheath layer.
Even after cross-linking of conventional polymers in accordance with the state of the art prior to the current invention, there has remained a need to improve the selectivity and permeability of the resulting membranes.
The present invention provides a new type of high selectivity, high plasticization-resistant and solvent-resistant, both chemically and UV cross-linked polyimide membranes.